*{
    padding: 0;
    margin: 0;
    box-sizing: border-box;
    list-style: none;
    text-decoration: none;
}
#calculator {
    width: 400px;
    margin: 10px auto;
    border: 1px solid #efefef;
    box-shadow: 4px 4px 8px 2px #999;
    /* box-shadow: 4px 4px 0 0 #ccc; */
    background: #efefef;
    padding: 30px 15px;
    box-sizing: border-box;
    border-radius: 10px;
}
.input-wrap {
    width: 100%;
}
.expression {
    line-height: 3em;
    height: 2.5em;
    white-space: nowrap;
    text-overflow: ellipsis;
    color: #666;
}
input {
    border: none;
    width: 100%;
    padding: 8px 10px;
    box-sizing: border-box;
    font-size: 16px;
    background-color: #b3d1c7;
    box-shadow: 1px 1px 0 0 #aaa inset;
    border-radius: 5px;
}
input:focus {
    border: none;
    outline: none;
}
#keyboard {
    display: flex;
    justify-content: space-between;
    align-items: center;
    flex-wrap: wrap;
}
#keyboard li {
    width: calc(25% - 7px);
    text-align: center;
    border: 1px solid #ddd;
    margin-top: 10px;
    line-height: 2.5em;
    border-radius: 5px;
    box-shadow: 2px 2px 0 0 #aaa;
}